Trees dying faster, scientists discover

Warming the likely culprit, says UBC professor who’s seen rate double in 17 years

By Gordon Hamilton, Vancouver Sun – January 23, 2009

On seven plots of ancient forest deep in the Capilano and Seymour watersheds, University of B.C. biogeography professor Lori Daniels has made an alarming discovery: The trees she is monitoring are dying off.

The death rate of the forest giants has, in fact, doubled since she first started studying them as a graduate student 17 years ago.

Now her finding is part of a comprehensive study of forests across western North American that links an increase in tree mortality to climate change.

Trees all across the West are dying at twice the rate they were 20 years ago, according to a report by Daniels and 10 other scientists to be published today in Science magazine.

“We are losing trees faster than we are gaining trees,” Daniels said in an interview Thursday.

In every instance, mortality rates have doubled, states the report, led by a U.S. Geological Survey team.

At the same time, average temperatures have climbed by one degree Celsius, making climate change the most likely cause, Daniels, an expert in old-growth forests, said.

The death rate is expected to continue to rise as temperatures go up, leading to sparser forests less able to act as carbon sinks, leading to even more warming.

The scientists say the trend is expected to continue.

Daniels studied more than 1,200 trees in old-growth forest plots on the North Shore. Beside noting the increasing death rate of older trees, she found that the undergrowth trees that would typically replace the old giants are suffering as well. They are not filling in the vacant spaces left when the veterans fall.

“It’s possible that if trees are stressed because of warmer temperatures causing them to die, the same stress is constraining their growth,” she said of evidence showing growth in younger trees is not as robust.

The U.S. Geological Survey team was put together by California scientist Phil van Mantgem, who noted increased mortality in Sequoia forests. He raised the issue with colleagues across the West, including Daniels, who had been monitoring their own plots for years, and the team was formed. The study, the largest of its kind in North America, gathered evidence from 76 plots in forests more than 200 years old.

The scientists conclude that tree death rates have doubled in 17 years in the coastal forests and in 29 years in Interior forests. Trees are dying across a wide variety of forest types, at all elevations. All sizes are dying and a variety of species, including pines, firs and hemlocks.

Mortality rates were less than one per cent a year when the scientists first began monitoring several decades ago, but are now two per cent, Daniels said.

“That might seem really small, but mortality rates work like interest in a bank account. They compound over time,” she said.

Over a 50-year period, out of 100 trees only 36 will survive at the current mortality rate, she said. Twenty years ago, 65 would survive.

That degree of change is alarming, she said.

Climate change is being pinpointed because the one-degree change in temperature means snow packs are smaller. That leads to longer dry periods in summer, when trees are stressed by drought, Daniels said.

“Increased temperature is going to change not only the metabolic rate in the trees, but their need for water. We have detected both an increase in temperature and an increase in the water deficit, which means there is a water shortage for these trees at the same time the temperature is going up,” Daniels said.

Warmer temperatures are also proving a boon to insect pests that are attacking and killing trees—from the mountain pine beetle, that’s devastated forests from northern B.C. to Colorado, to the tiny Western hemlock looper, which was responsible for a significant number of tree deaths in and around Daniels’ old-growth plots.

The findings of the team are not isolated. Researchers in B.C. have found increased tree mortality elsewhere. The obvious example is the Interior, where warmer winters are being blamed for the massive mountain pine beetle outbreak.

But on the B.C. coast in 2004, ministry of forests researchers began documenting an increasing death rate among yellow cedars. In the last four years, they have found more than 47,000 hectares of dying trees from the Alaska border to Kingcome Inlet. Insect pests and fungi have been ruled out, forest pathologist Stefan Zeglen said. They suspect a warmer climate has led to decreased snow packs. Snow disappears earlier in the year and exposes fragile tree roots to damaging spring frosts.